1. Field of the Invention
The present invention relates to a zoom lens capable of changing a focal length by moving lens groups along an optical axial direction back and forth and an image-taking device equipped with the zoom lens, more specifically to a zoom lens especially suitable for a digital camera, a portable terminal device, a video camera and or the like and an image-taking device equipped with the zoom lens.
In recent years, the digital camera having a changeable focus mechanism such as a zoom lens camera barrel or the like is commonly available. The changeable focus mechanism can change continuously between a wide angle state and a telescopic angle state. The wide-angle state is that a focal length of a photographing lens is short and an angle of field is wide. The telescopic angle state is that the focal length of the photographing lens is long and the angle of field is narrow.
As for the digital cameras in recent years, a better picture, miniaturizing, more of the wide angle, more of a large caliber, and or the like are desired in a market.
Moreover, the better picture, miniaturizing, more of the wide angle and more of the large caliber for corresponding to a light detecting element which exceeds three million pixels are requested. The miniaturization means reducing a storage length of the lens, a total length of the lens, and a maximum of an effective diameter or the like.
2. Description of the Related Art
So, as a technical document filed earlier than the present invention, for example, Japanese Patent Publication No. 2004-354870 discloses a zoom lens of rear focus type which includes as follows: a first lens group that has a positive refracting power; a second lens group that has a negative refracting power; a third lens group that has a positive refracting power; a fourth lens group that has a positive refracting power; a fifth lens group that has a negative refracting power. The zoom lens is configured to zoom by moving the second lens group and the fourth lens group. When a distance of object is infinity and an image formation magnification of the fifth lens group is set to be β5 simultaneously, the zoom lens satisfies a condition of 1.6<β5<2.2. An excellent optical performance is obtained if a zooming ratio is set about 3-5 times. An entire system of the lens is miniaturized.
Moreover, Japanese Patent Publication No. 2004-286811 discloses a zoom lens that includes as follows sequentially from an object side of the zoom lens to an image side of the zoom lens: a first lens group that has a positive refracting power and doesn't move along with a zooming; a second lens group which has a negative refracting power and moves from the object side of the zoom lens to the image side of the zoom lens along with the zooming from a wide angle end to a telephoto end; a third lens group that has a positive refracting power and moves from the image side of the zoom lens to the object side of the zoom lens from the wide angle end to the telephoto end along with the zooming; a fourth lens group that has a positive refracting power and doesn't move along with the zooming. The third lens group has a three piece cemented lens that consists of a negative lens, a positive lens and a negative lens. Even though the zoom lens is small enough with a wide angle of field, the zoom lens is efficient, and the lens enables it to obtain a resolving power corresponding to an image sensor of 3-5 million pixels.
Moreover, Japanese Patent Publication No. 2004-199000 discloses a zoom lens that includes as follows: from an object side of the zoom lens sequentially, a first lens group that has a positive refracting power; a second lens group that has a negative refracting power; a third lens group that has a positive refracting power; a fourth lens group that has a positive refracting power. The first lens group consists only of one positive lens. During zooming from a wide angle end to a telephoto end, all the lens groups are moved in such a way that an interval between the first lens group and the second lens group increases, an interval between the second lens group and the third lens group decreases and an interval between the third lens group and the fourth lens group increases. At the same time, the third lens group and the fourth lens group are moved in a direction to the object side of the zoom lens. The zoom lens satisfies following conditional expressions: 5.0<f1/fw<9.5, −2.2<f2/fw<−1.2 and 0.4<f3/f4<1.5 (fw: a focal length of all zoom lens groups in a state of the wide angle end. F1: a focal length of the first lens group. F2: a focal length of the second lens group. F3: a focal length of the third lens group. F4: a focal length of the fourth lens group.). The zoom lens is suitable for a video camera or the like using a solid state image sensor or the like. The zoom lens is small and has a zoom factor about three times. In the zoom lens, an angle of field in the state of the wide angle end is 60 degrees or more and F number is about F2-F2.8. So, the zoom lens has an excellent image formation performance.
Moreover, Japanese Patent Publication No. 2005-181556 discloses a large caliber zoom lens that includes from an object side of the zoom lens sequentially as follows: a first lens group with a positive refracting power; a second lens group with a negative refracting power; a third lens group with a positive refracting power; and a fourth lens group with a positive refracting power. By a zooming, an interval between the first lens group and the second lens group extends, an interval between the second lens group and the third lens group narrows, and an interval between the third lens group and the fourth lens group narrows. The second lens group is moved to the object side of the zoom lens in a focusing. The lens satisfies following conditional expressions: (1) 0.5<f2/fW<1.0; (2) 0.5<f3/fT<2.0; (3) 0.5<f4/fW<2.0; (fW: a focal length of all optical systems in a wide angle end side. fT: the focal length of all optical systems in a telephoto side. f2: a focal length of the second lens group. f3: a focal length of the third lens group. f4: a focal length of the fourth lens group).
It is possible for the zoom lens disclosed in patent document 1 to correspond to the wide angle of field where the half field angle is about 38 degrees, but not to 2.5 or less in F number in a short focal end, and the first lens group is a thick structure. The 38 degrees in the half field angle corresponds to 28 mm of a focal length in a 35 mm silver bromide film camera, so-called Leica seal camera.
The zoom lenses disclosed in the patent documents 2 and 4 can correspond to the wide angle of field where the half field angle is 38 degrees or more, but can not correspond to 2.5 or less in F number of the short focal ends.
The zoom lens disclosed in above-mentioned patent document 3 can correspond to 2.5 or less in F number of the wide angle ends, but can not correspond to the wide angle of field where the half field angle is 38 degrees or more.